Two terminal memory array having reference cells

ABSTRACT

A memory including reference cells is provided. The memory has address decoding circuitry and an array of memory cells that are non-volatile and re-writable. Each memory cell has a two terminal memory plug that is capable of experiencing a change in resistance. Sensing circuitry compares activated memory cells to a reference level. The reference level is typically generated by at least one reference cell that can be selected at the same time the memory cell is selected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to re-writeable non-volatile computermemory having very small feature sizes.

2. Description of the Related Art

Memory can either be classified as volatile or non-volatile. Volatilememory is memory that loses its contents when the power is turned off.In contrast, non-volatile memory does not require a continuous powersupply to retain information. Most non-volatile memories use solid-statememory devices as memory elements.

Since the 1960s, a large body of literature has evolved that describesswitching and memory effects in metal-insulator-metal structures withthin insulators. One of the seminal works was “New Conduction andReversible Memory Phenomena in Thin Insulating Films” by J. G. Simmonsand R. R. Verderber in 301 Proc. Roy. Soc. 77-102 (1967), incorporatedherein by reference for all purposes. Although the mechanisms describedby Simmons and Verderber have since been cast into doubt, theircontribution to the field is great.

However, no one has successfully implemented a metal-insulator-metalstructure into a commercial solid-state memory device. In the text“Oxides and Oxide Films,” volume 6, edited by A. K. Vijh (Marcel Drekker1981) 251-325, incorporated herein by reference for all purposes,chapter 4, written by David P. Oxley, is entirely devoted to “MemoryEffects in Oxide Films.” In that text, Oxley says “It is perhapssaddening to have to record that, even after 10 years of effort, thenumber of applications for these oxide switches is so limited.” He goeson to describe a “need for caution before any application is envisaged.This caution can only be exercised when the physics of the switchingaction is understood; this, in turn, must await a full knowledge of thetransport mechanisms operating in any switch for which a commercial useis envisaged.”

In 2002, over twenty years after writing that chapter, Oxley revisitedthe subject in “The Electroformed metal-insulator-metal structure: Acomprehensive model” by R. E. Thurstans and D. P. Oxley, 35 J. Phys. D.Appl. Phys. 802-809, incorporated herein by reference for all purposes.In that article, the authors describe a model that identifies theconduction process as “trap-controlled and thermally activated tunnelingbetween metal islands produced in the forming process.” “Forming” (or“electroforming”) is described as “the localized filamentary movement ofmetallic anode material through the dielectric, induced by the electricfield. Here it is important to note that the evaporated dielectric maycontain voids and departures from stoichiometry. When resultingfilaments through the dielectric carry sufficient current, they ruptureto leave a metal island structure embedded in the dielectric. Electronicconduction is possible through this structure by activating tunneling.”

However, the authors caution, “The forming process is complex andinherently variable. Also tunneling barriers are susceptible to changesin their characteristics when exposed to water vapour, organic speciesand oxygen . . . Thus, device characteristics can never be expected tobe produced consistently or be stable over long periods withoutpassivation, effective encapsulation and a better understanding of thedynamics of the forming process.”

In seemingly unrelated research, certain conductive metal oxides (CMOs),have been identified as exhibiting a memory effect after being exposedto an electronic pulse. U.S. Pat. No. 6,204,139, issued Mar. 20, 2001 toLiu et al., incorporated herein by reference for all purposes, describessome perovskite materials that exhibit memory characteristics. Theperovskite materials are also described by the same researchers in“Electric-pulse-induced reversible resistance change effect inmagnetoresistive films,” Applied Physics Letters, Vol. 76, No. 19, 8 May2000, and “A New Concept for Non-Volatile Memory: The Electric-PulseInduced Resistive Change Effect in Colossal Magnetoresistive ThinFilms,” in materials for the 2001 Non-Volatile Memory TechnologySymposium, all of which are hereby incorporated by reference for allpurposes.

In U.S. Pat. No. 6,531,371 entitled “Electrically programmableresistance cross point memory” by Hsu et al, incorporated herein byreference for all purposes, resistive cross point memory devices aredisclosed along with methods of manufacture and use. The memory devicecomprises an active layer of perovskite material interposed betweenupper electrodes and lower electrodes.

Similarly, the IBM Zurich Research Center has also published threetechnical papers that discuss the use of metal oxide material for memoryapplications: “Reproducible switching effect in thin oxide films formemory applications,” Applied Physics Letters, Vol. 77, No. 1, 3 Jul.2000, “Current-driven insulator-conductor transition and non-volatilememory in chromium-doped SrTiO₃ single crystals,” Applied PhysicsLetters, Vol. 78, No. 23, 4 Jun. 2001, and “Electrical currentdistribution across a metal-insulator-metal structure during bistableswitching,” Journal of Applied Physics, Vol. 90, No. 6, 15 Sep. 2001,all of which are hereby incorporated by reference for all purposes.

There are continuing efforts to incorporate solid state memory devicesinto a commercial non-volatile RAM.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1A depicts a perspective view of an exemplary cross point memoryarray employing a single layer of memory;

FIG. 1B depicts a perspective view of an exemplary stacked cross pointmemory array employing four layer of memory;

FIG. 2A depicts a plan view of selection of a memory cell in the crosspoint array depicted in FIG. 1A;

FIG. 2B depicts a perspective view of the boundaries of the selectedmemory cell depicted in FIG. 2A;

FIG. 3 depicts a generalized cross-sectional representation of a memorycell that can be used in a transistor memory array;

FIG. 4A depicts a block diagram of a representative implementation of anexemplary 1 MB memory;

FIG.4B depicts a block diagram of an exemplary memory that includessensing circuits that are capable of reading multiple bits;

FIG. 5A depicts a block diagram of an exemplary memory array that has asingle reference cell; and

FIG. 5B depicts a block diagram of an exemplary memory array that has areference column.

It is to be understood that, in the drawings, like reference numeralsdesignate like structural elements. Also, it is understood that thedepictions in the FIGS. are not necessarily to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, numerous specific details are set forth toprovide a thorough understanding of the present invention. It will beapparent, however, to one skilled in the art that the present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process steps have not been described in detail inorder to avoid unnecessarily obscuring the present invention.

The Memory Array

Conventional non-volatile memory requires three terminal MOSFET-baseddevices. The layout of such devices is not ideal, usually requiring anarea of at least 8f² for each memory cell, where f is the minimumfeature size. However, not all memory elements require three terminals.If, for example, a memory element is capable of changing its electricalproperties (e.g., resistivity) in response to a voltage pulse, only twoterminals are required. With only two terminals, a cross point arraylayout that allows a single cell to be fabricated to a size of 4t² canbe utilized.

FIG. 1A depicts a perspective view of an exemplary cross point memoryarray 100 employing a single layer of memory. A bottom layer ofx-direction conductive array lines 105 is orthogonal to a top layer ofy-direction conductive array lines 110. The x-direction conductive arraylines 105 act as a first terminal and the y-direction conductive arraylines 110 act as a second terminal to a plurality of memory plugs 115,which are located at the intersections of the conductive array lines 105and 1 10. The conductive array lines 105 and 110 are used to bothdeliver a voltage pulse to the memory plugs 115 and carry currentthrough the memory plugs 115 in order to determine their resistivestates.

Conductive array line layers 105 and 110 can generally be constructed ofany conductive material, such as aluminum, copper, tungsten or certainceramics. Depending upon the material, a conductive array line wouldtypically cross between 64 and 8192 perpendicular conductive arraylines. Fabrication techniques, feature size and resistivity of materialmay allow for shorter or longer lines. Although the x-direction andy-direction conductive array lines can be of equal lengths (forming asquare cross point array) they can also be of unequal lengths (forming arectangular cross point array), which may be useful if they are madefrom different materials with different resistivities.

FIG. 2A illustrates selection of a memory cell 205 in the cross pointarray 100. The point of intersection between a single x-directionconductive array line 210 and a single y-direction conductive array line215 uniquely identifies the single memory cell 205. FIG. 2B illustratesthe boundaries of the selected memory cell 205. The memory cell is arepeatable unit that can be theoretically extended in one, two or eventhree dimensions. One method of repeating the memory cells in thez-direction (orthogonal to the x-y plane) is to use both the bottom andtop surfaces of conductive array lines 105 and 110, creating a stackedcross point array.

FIG. 1B depicts an exemplary stacked cross point array 150 employingfour memory layers 155,160,165, and 170. The memory layers aresandwiched between alternating layers of x-direction conductive arraylines 175, 180 and 185 and y-direction conductive array lines 190 and195 such that each memory layer 155, 160, 165, and 170 is associatedwith only one x-direction conductive array line layer and oney-direction conductive array line layer. Although the top conductivearray line layer 185 and bottom conductive array line layer 175 are onlyused to supply voltage to a single memory layer 155 and 170, the otherconductive array line layers 180, 190, and 195 can be used to supplyvoltage to both a top and a bottom memory layer 155, 160, 165, or 170.

Referring back to FIG. 2B, the repeatable cell that makes up the crosspoint array 100 can be considered to be a memory plug 255, plus ½ of thespace around the memory plug, plus ½ of an x-direction conductive arrayline 210 and ½ of a y-direction conductive array line 215. Of course, ½of a conductive array line is merely a theoretical construct, since aconductive array line would generally be fabricated to the same width,regardless of whether one or both surfaces of the conductive array linewas used. Accordingly, the very top and very bottom layers of conductivearray lines (which use only one surface) would typically be fabricatedto the same size as all other layers of conductive array lines.

One benefit of the cross point array is that the active circuitry thatdrives the cross point array 100 or 150 can be placed beneath the crosspoint array, therefore reducing the footprint required on asemiconductor substrate. However, the cross point array is not the onlytype of memory array that can be used with a two-terminal memoryelement. For example, a two-dimensional transistor memory array canincorporate a two-terminal memory element. While the memory element insuch an array would be a two-terminal device, the entire memory cellwould be a three-terminal device.

FIG. 3 is a generalized diagrammatic representation of a memory cell 300that can be used in a transistor memory array. Each memory cell 300includes a transistor 305 and a memory plug 310. The transistor 305 isused to permit current from the data line 315 to access the memory plug310 when an appropriate voltage is applied to the select line 320, whichis also the transistor's gate. The reference line 325 might span twocells if the adjacent cells are laid out as the mirror images of eachother.

Memory Chip Configuration

FIG. 4A is a block diagram of a representative implementation of anexemplary 1 MB memory 400A. Physical layouts might differ, but eachmemory bit block 405 can be formed on a separate portion of asemiconductor substrate. Input signals into the memory 400A can includean address bus 430, a control bus 440, some power supplies 450, and adata bus 460. The control bus 440 typically includes signals to selectthe chip, to signal whether a read or write operation should beperformed, and to enable the output buffers when the chip is in readmode. The address bus 430 specifies which location in the memory arrayis accessed—some addresses going to the X block 470 (typically includinga predecoder and an X-decoder) to select one line out of the horizontalarray lines. The other addresses go to a Y block 480 (typicallyincluding a predecoder and a Y-decoder) to apply the appropriate voltageon specific vertical lines. Each memory bit block 405 operates on oneline of the memory chip data bus 460.

The reading of data from a memory array 420 is relativelystraightforward: an x-line is energized, and current is sensed by thesensing circuits 410 on the energized y-lines and converted to bits ofinformation. FIG. 4B is a block diagram of an exemplary memory 400B thatincludes sensing circuits 415 that are capable of reading multiple bits.The simultaneous reading of multiple bits involves sensing current frommultiple y-lines simultaneously.

During a write operation, the data is applied from the data bus 460 tothe input buffers and data drivers 490 to the selected vertical lines,or bit lines. Specifically, when binary information is sent to thememory chip 400B, it is stored in latch circuits within the circuits490. Each y-line can either have an associated driver circuit 490 or agroup of y-lines can share a single driver circuit 490 if thenon-selected lines in the group are held to a constant voltage thatwould not cause the unselected memory plugs to experience any change inresistance. The driver circuit then writes the 1 or 0 to the appropriatememory plug during the appropriate cycle. For example, there may be 1024y-lines in a cross point array, and the page register may include 8latches, in which case the y-block would decode 1 out of 128 y-lines andconnect this selected line to block 490. As described below, certainmemory plugs can have multiple stable distinct resistive states. Withsuch multi-level resistance memory plugs, driver circuits could program,for example, states of 00, 01, 10 or 11 by varying write voltagemagnitude or pulse length.

It is to be noted that such an architecture can be expanded to create amemory where one array handles all the bits of the data bus, as opposedto having multiple arrays, or memory bit blocks as described above. Forexample, if the data bus, or memory data organization, also called datawidth, is 16-bit wide, the y-block of one cross point array can be madeto decode 16 lines simultaneously. By applying the techniques ofsimultaneous reads and 2-cycle writes, such a memory chip with only onearray can read and program 16-bit words.

Reference Cells

Reference cells can improve the performance of both a cross point memoryarray and a transistor memory array. FIG. 5A is a block diagram of anexemplary memory array 500 using a single reference cell 505. Referencecells provide a reference level to the system. Since the reference levelis known, the resistance state of other memory cells can be determinedby comparing their resistances to the known reference levels.

A simple way to provide a reference would be to build a resistance usinga diffusion layer, polysilicon layer or any device having a knownresistance. Alternatively, a resistance level could be achieved with aknown current source and a know voltage source, since the resistance isa voltage over a current. However, to better track the actual memorycells and compensate for fabrication and environmental differences, itis preferable to use the same type of material in the reference cell 505that is used in the memory cells of the memory array. Moreover, evenbetter uniformity can be achieved if the reference cell 505 were notjust the same type of material, but if it were actually fabricated tohave the same structure as every other memory cell of the memory array.

Various techniques could be used to set the resistance of the referencecell 505 to a known value. For example, special circuits could be usedto program the reference cell to the desired value. In the case ofbinary memory cells that are only programmed to either a high state or alow state, it is sometimes advantageous to have the reference cellprogrammed to a mid-point in-between the states. Sensing could beachieved very quickly because the system would not need to wait untilthe sensing circuits settled on a high value or a low value, but insteadcould trigger a high or low signal depending upon whether the sensingcircuit signal started to rise from the reference signal to the highsignal or fall from the reference signal to the low signal.

However, in a binary system special circuits would generally be requiredto program the reference cell 505 to a middle state since the otherarray circuits would be designed to only program memory cells to a highstate or a low state. Special circuitry would not be required if thereference cell came out of fabrication with the appropriate resistance,either by inherently having the correct resistance or by implementingvarious processing techniques to program the reference cell duringfabrication. Special circuitry would also not be required if tworeference cells were used. If one was programmed to a high resistivestate and the other was programmed to a low resistive state, then thetwo cells could be electrically combined to produce an appropriateequivalent resistance. Additionally, reference cells programmed to ahigh resistive state could be used as a “write high verify” to ensurememory cells are programmed correctly. Reference cells programmed to alow resistive state could be used as a “write low verify” in a similarmanner. Although these write verify reference cells would preferably bethe same reference cells that are combined and used in a read, separatereference columns could be used for read and write operations. Redundantcells could also be combined so that any effects from fabricationdefects of a single cell would be minimized. For simplicity, a“reference cell” is described herein as being singular, but can includemultiple reference cells that are combined to produce an appropriatereference level.

It is often more convenient and more accurate to employ a column ofreference cells instead of one reference cell 505 for the entire array.In this manner, each row would contain its own reference cell. FIG. 5Bis a block diagram of an exemplary memory array 550 using a referencecolumn 555. Although the reference column 555 is shown as being locatednear the center of the array, it can be placed anywhere on the array.However, placing the reference column 555 at the center provides it withboth a closer match in physical characteristics to other cells in thearray, eliminates any edge effects, and reduces the distance of theconductive line path from edge columns to the reference column 555.

In the arrangement of FIG. 5B, an X-line 560 can be energized to apply aread voltage to all of its memory cells, including its associatedreference cell such that all the devices on the X-line shared the samevoltage. The resulting voltages from any energized Y-lines are thensensed by a sense amp or plurality of sense amps. A sense amp is anycomponent or group of components for determining the current or voltageof a memory cell relative to the reference level of the reference cell.In one embodiment, each Y-line is placed in electrical communicationwith an input of a differential amplifier, with the other input beingthe voltage from the reference column 555. The selected Y-line(s) andthe reference column are biased to induce a current flow in theassociated memory plugs of the memory cell(s) and reference cell. Whenthis technique is used the current in the selected cell is read on theselected y-line in parallel with the reference column in line 555. Theoutput of each differential amplifier thus measures the difference involtage between the output voltage of each memory cell and the outputvoltage of the reference cell. Those skilled in the art will appreciatethat sensing can also be accomplished through current sensing.

Memory Plug

Each memory plug contains layers of materials that may be desirable forfabrication or functionality. For example, a non-ohmic characteristicthat exhibit a very high resistance regime for a certain range ofvoltages (V_(NO−) to V_(NO+)) and a very low resistance regime forvoltages above and below that range might be desirable. In a cross pointarray, a non-ohmic characteristic could prevent leakage during reads andwrites if half of both voltages were within the range of voltagesV_(NO−) to V_(NO+). If each conductive array line carried ½ V_(W), thecurrent path would be the memory plug at the intersection of the twoconductive array lines that each carried ½ V_(W). The other memory plugswould exhibit such high resistances from the non-ohmic characteristicthat current would not flow through the half-selected plugs.

A non-ohmic device might be used to cause the memory plug to exhibit anon-linear resistive characteristic. Exemplary non-ohmic devices includethree-film metal-insulator-metal (MIM) structures and back-to-backdiodes in series. Separate non-ohmic devices, however, may not benecessary. Certain fabrications of the memory plug can cause a non-ohmiccharacteristic to be imparted to the memory cell. While a non-ohmiccharacteristic might be desirable in certain arrays, it may not berequired in other arrays.

Electrodes will typically be desirable components of the memory plugs, apair of electrodes sandwiching the memory element. If the only purposeof the electrodes is as a barrier to prevent metal inter-diffusion, thena thin layer of non-reactive metal, e.g. TiN, TaN, Pt, Au, and certainmetal oxides could be used. However, electrodes may provide advantagesbeyond simply acting as a metal inter-diffusion barrier. Electrodes(formed either with a single layer or multiple layers) can performvarious functions, including: prevent the diffusion of metals, oxygen,hydrogen and water; act as a seed layer in order to form a good latticematch with other layers; act as adhesion layers; reduce stress caused byuneven coefficients of thermal expansion; and provide other benefits.Additionally, the choice of electrode layers can affect the memoryeffect properties of the memory plug and become part of the memoryelement.

Memory Effect

The memory effect is a hysteresis that exhibits a resistive state changeupon application of a voltage while allowing non-destructive reads. Anon-destructive read means that the read operation has no effect on theresistive state of the memory element. Measuring the resistance of amemory cell is generally accomplished by detecting either current afterthe memory cell is held to a known voltage, or voltage after a knowncurrent flows through the memory cell. Therefore, a memory cell that isplaced in a high resistive state R₀ upon application of −V_(W) and a lowresistive state R₁ upon application of +V_(W) should be unaffected by aread operation performed at −V_(R) or +V_(R). In such materials a writeoperation is not necessary after a read operation. It should beappreciated that the magnitude of |−V_(R)| does not necessarily equalthe magnitude of |+V_(R)|.

Furthermore, it is possible to have a memory cell that can be switchedbetween resistive states with voltages of the same polarity. Forexample, in the paper “The Electroformed metal-insulator-metalstructure: a comprehensive model,” (already incorporated by reference)Thurstans and Oxley describe a memory that maintains a low resistivestate until a certain V_(P) is reached. After V_(P) is reached theresistive state can be increased with voltages. After programming, thehigh resistive state is then maintained until a V_(T) is reached. TheV_(T) is sensitive to speed at which the program voltage is removed fromthe memory cell. In such a system, programming R₁ would be accomplishedwith a voltage pulse of V_(P), programming R₀ would be accomplished witha voltage pulse greater than V_(P), and reads would occur with avoltages below V_(T). Intermediate resistive states (for multi-levelmemory cells) are also possible.

The R₁ state of the memory plug may have a best value of 10 kΩ to 100kΩ. If the R₁ state resistance is much less than 10 kΩ, the currentconsumption will be increased because the cell current is high, and theparasitic resistances will have a larger effect. If the R₁ state valueis much above 100 kΩ, the RC delays will increase access time. However,workable single state resistive values may also be achieved withresistances as low as 5 kΩ and as high as 1 MΩ. Typically, a singlestate memory would have the operational resistances of R₀ and R₁separated by a factor of 10.

Since memory plugs can be placed into several different resistivestates, multi-bit resistive memory cells are possible. Changes in theresistive property of the memory plugs that are greater than a factor of10 might be desirable in multi-bit resistive memory cells. For example,the memory plug might have a high resistive state of R₀₀, a medium-highresistive state of R₀₁, a medium-low resistive state of R₁₀ and a lowresistive state of R₁₁. Since multi-bit memories typically have accesstimes longer than single-bit memories, using a factor greater than a 10times change in resistance from R₁₁ to R₀₀ is one way to make amulti-bit memory as fast as a single-bit memory. For example, a memorycell that is capable of storing two bits might have the low resistivestate be separated from the high resistive state by a factor of 100. Amemory cell that is capable of storing three or four bits of informationmight require the low resistive state be separated from the highresistive state by a factor of 1000.

Although the memory effect properties of the memory plug appear to bedominated by carrier trapping, other carrier transport mechanisms suchas oxygen migration or electrolyte migration may be present. Even withincarrier charge trapping, dominant factors can include space-chargelimited currents, thermionic emission limited conduction, electrothermalPoole-Frenkel emission, or Fowler-Nordheim quantum tunneling. While theinventors note that experimental data is consistent with memory effectsbeing created primarily by carrier trapping, they do not wish to bebound by any explanation of how a memory effect is created or how anyother effect that is described herein functions.

As previously described, forming is one technique that can be used inmetal-insulator-metal structures in order to induce a memory effect.However, it is generally not practical to form a structure within acommercial memory product. Therefore, processing techniques are requiredto either eliminate the need for forming or create conditions that makeforming possible in high-volume manufacturing.

Creating the Memory Effect

Interfacial layers are one mechanism that can be used to create a memoryeffect. An interface layer is typically a very thin layer because thereaction that is created by placing the oxide in contact with thereactive metal only extends a short distance, typically less than 100angstroms. The thickness of the interfacial layer can be controlled bylimiting the thickness of the reactive metal. Although the interfaciallayer can be placed into different resistive states, even the lowestresistive state is typically very insulating. Accordingly, a thickinterfacial layer would not allow any current to pass through the memorycell in an optimal period of time. To allow rapid access times (on theorder of tens of nanoseconds, typically below 100 ns) in small dimensiondevices (on the order of hundreds of nanometers), the entire memory plugshould have a resistivity of not more than about 1 ohm-cm.

The oxide will generally (but not necessarily) be a conductivecrystalline metal oxide—either as a single crystalline structure or apolycrystalline structure. One class of conductive oxides areperovskites that include two or more metals, the metals being selectedfrom the group consisting of transition metals, alkaline earth metalsand rare earth metals.

The perovskites (generally in the form of ABX₃ structures, where A hasan atomic size of 1.0-1.4 Å and B has an atomic size of 0.45-0.75 Å forthe case where X is either oxygen or fluorine) can be any number ofcompositions, including manganites (e.g., Pr_(0.7)Ca_(0.3)MnO₃,Pr_(0.5)Ca_(0.5)MnO₃ other PCMOs, LCMOs using lanthanum and calcium asA, etc.), titanates (e.g., SrTiO₃), and zirconates (e.g., SrZrO₃). Someperovskites can be doped with various elements, such as Nb, in order tomake them less insulating. MnO₃, when combined with the rare earthmetals La, Pr or some combination thereof and the alkaline earth metalsCa, Sr or some combination thereof have been found to be particularlyeffective for use in the memory plug.

Further, some oxides that may not be conductive in their pure form maybe used as they become conductive through the addition of dopants, or ifthey are used as a very thin layer (e.g., in the order of tens ofAngstroms) in which case tunneling conduction can be achieved.Therefore, as will be appreciated by those skilled in the art, oxidesthat are classified as insulators, but are thin enough to allowtunneling conduction, can still be considered conductive oxides. Since amemory plug will need to be able to be switched at low currents, lowresistances are desirable—making conductive oxides more attractive thaninsulating oxides.

Whether a metal is a reactive metal is determined by its relation to theperovskite, and whether the metal is a strong enough reducing agent toreduce the semiconductive metal oxide and be oxidized. For example, Alwill reduce a PCMO perovskite. Other reactive metals, depending on thesemiconductive metal oxide, can include Ta, Ti, Zr, Y, Hf, Cr and Mg. Itshould be appreciated that the resulting interfacial layer may not havewell-defined stoichiometry, but can be more like a “soup” of Al, Pr, Caand Mn oxides.

In one embodiment, the interfacial layer can be considered to be betweenthe unaltered conductive oxide and the reactive metal (or reactive metaloxide, depending on the deposition technique) that does not contain anyconductive oxide. If, however, only a small amount of reactive metal isdeposited such that all of the reactive metal is reacted, then theinterfacial layer will be between the unaltered conductive oxide and anelectrode on top of the interfacial layer.

However, it should be appreciated that using a reactive metal may not bethe only way to create a memory effect with a conductive oxide.Insulating and semiconductive layers having the memory effect might beable to be created through doping, implantation or use of othertechniques and treatments.

For example, strontium titanate (STO) or strontium zirconate (SZO) canbe doped by adding an element that has a different preferred oxidationstate (a different charge when ionized) when it replaces an element inthe crystal matrix. Typically, the dopant would make up less than 10%molecular percentage of the total material. In SZO, chromium (Cr), witha +3 oxidation state, can replace zirconium (Zr) with a +4 oxidationstate. Charge imbalance is compensated by either creation of appropriatevacancies (e.g., oxygen vacancies), by change of valence on a matrixelement, or by introduction of free carriers (electrons or holes).

A dopant atom usually substitutes for a matrix element based, at leastpartly, on the similarity of ionic radii. Thus lanthanum (La) primarilysubstitutes for strontium (Sr) whereas Cr primarily substitutes for Zrin SZO. In SZO, cation vacancies are rare (i.e., there are few Sr and Zrvacancies), but anion vacancies (i.e., oxygen) are common. Therefore,adding Cr to SZO generates both oxygen vacancies and free holes.However, Cr addition is usually compensated by oxygen vacancies (onevacancy for every two Cr atoms) such that the material remainsessentially insulating. In contrast, free electrons primarily compensatefor La in SZO. Therefore adding La drastically lowers the resistance ofSZO. Similarly tantalum (Ta) or niobium (Nb) can substitute for Zr tolower SZO resistivity.

Further, vacancies (either anion or cation) can also act to createcharge traps. The charge imbalance caused by a vacancy can becompensated by the same mechanisms that compensate for the intentionaladditions of a dopant. Thus, an oxygen vacancy compensated by 2 Cr atomsprovides no free carriers, but if there is insufficient Cr for fullcompensation, oxygen vacancies lead to free electrons.

Some dopants create centers at deep levels in the bandgap. Such dopantscreate centers where a charge would need a high level of energy to exitthat level, effectively creating traps with the deep levels. Forexample, Cr, iron (Fe),or nickel (Ni) can create traps in STO and SZO.To the contrary, yttrium (Y), La, Nb and Ta would create centers atshallow levels, which would not be traps.

Treatment can additionally occur through, for example, ion implantation.In ion implantation accelerated ions penetrate a solid surface up tocertain depth that is determined by the ion energy. Ion implantation canbe used to introduce dopants, to form buried layers, and to modify solidsurfaces.

Another treatment method is to expose a reactive metal or a conductiveoxide to either an anneal or a gas at a given temperature within a givenambient. Some anneals can be easily integrated into fabrication. Forexample, if the array only has a single memory plug layer, then thebottom layers might be subjected to high temperatures in order toproperly form the conductive oxide. However, the top layers can then bedeposited at temperatures far below what is necessary for forming theconductive oxide. Similar results can be obtained by laser treating oneof the surfaces, or exposing one of the surfaces to a plasma process(such as plasma etching).

Another treatment method might be to expose the entire structure and/ora particular surface layer to a physical re-sputtering, typically byusing Ar and/or O₂ or other inert gas plasma. Re-sputtering is atechnique commonly used to clean-up surfaces. Since a new film is notdeposited when the plasma hits the surface in the sputtering chamber, itcan be considered to be the opposite of sputtering. Similarly, thesurface can be exposed to an inert ion from an ion gun, bombarding thesurface with accelerated inert ions, such as ionized Ar.

Typically, the goal of such treatments is to create traps. Traps canalso be introduced with high energy radiation or particle beambombardment. For example, UV and X-ray radiation induces traps in SiO₂.Also, neutron transmutation doping can be used to create dopant atoms insilicon. Furthermore, traps can be created by an electricalinitialization process during which oxygen vacancies drift through anoxide in the presence of an applied electric field. Of course, thedominant carrier mechanisms may change depending upon the treatment ofthe interface layer.

A memory plug, therefore, has many similarities with a traditional MIMstructure. However, it should be noted that many memory plugs exhibitmemory characteristics regardless of whether they have been formed. Itshould be appreciated that the term “memory element” includes all thelayers that contribute to the memory effect. Such layers, depending onthe specific embodiment, can include the interface layer, the conductiveoxide, reactive metal layer and/or electrodes.

Depositing conductive islands of one material within the bulk of asecond, less conductive material is another way of creating the memoryeffect. Islands (as opposed to a continuous thin film) can be formed bya number of different processes, including sputtering, co-sputtering,evaporation, molecular beam epitaxy, atomic layer deposition,implantation, etc, and are typically related to the surface energies ofthe two materials. Those skilled in the art will appreciate that a firstmaterial can form islands on the surface of a second material under someprocesses and not others.

The density of the islands will typically be less than the percolationthreshold (the point at which a continuous path of nearest neighborscould be formed from one side to another). Once the percolationthreshold is reached, the layer becomes more like a thin film and lesslike an island structure. The size of the islands will typically benano-structures, between 0.5 and 50 nanometers thick.

In one specific embodiment, a portion of a semiconductive material isfirst deposited on an electrode. Then, conductive islands are formed onthe semiconductive material. After the islands are formed, anotherportion of the same semiconductive material is deposited on the islands.Then, either a top electrode is formed or additionalisland/semiconductive material layers are formed prior to the topelectrode.

Conductive metal islands can then be formed by depositing any number ofdifferent materials on the surface of the perovskite. A reactive metal,for example, will create a strong memory effect. However, a metal havinga purely ohmic connection with the perovskite will generally not haveany memory effect.

In another specific example, a damascene process could be employed wherean interlayer dielectric such as SiO₂ is patterned in order to createvoids over the bottom electrodes. Aluminum oxide can then be depositedand polished off the SiO₂ surface such that a portion of the void isfilled with aluminum oxide. A small amount of conductive metal or oxidecould then be formed on top of the aluminum oxide, which willpreferentially nucleate into an array of island formations on thealuminum oxide. Another layer of aluminum oxide could then be sputteredto fill more the void, followed by another polish and then anotherdeposition of islands. A final layer of aluminum oxide could then bedeposited to completely fill the void, followed by a final polish sothat the SiO₂/aluminum oxide surface was smooth.

It should also be appreciated that a reactive metal may not be creatingthe semiconductive metal islands (for example, AlO₂ may be moreinsulating than the PCMO perovskite, but the remaining elements of Pr,Ca and Mn may themselves form a conductive island). While this is onepossible explanation of the reaction that occurs within the variousmaterials, the inventors do not wish to be bound by any explanation ofhow conductive islands are formed when a reactive metal is deposited ona semiconductive metal oxide.

Another possible explanation is that the conductive islands aresurrounded by an insulating envelope. If such a reaction were occurringin the above example, then the islands would consist of conductive Alcores surrounded by an insulating skin of AlO₂, embedded within thesemiconductive PCMO perovskite.

Alternatively, a memory effect can be created with a non-reactive metal,such as gold, silver or platinum. The non-reactive metal would not reactwith the underlying perovskite, but instead (depending on surfaceenergies) simply preferentially nucleate into an array of conductiveisland structures. Although such non-reactive metals may not create asstrong a memory effect, stronger memory effects can be created byincreasing the size of the islands, creating multiple layers of islands,and/or modifying the Schottky barrier between the metal and thesurrounding semiconductor.

Filamentary forming is yet another mechanism than could be used toinduce a memory effect. Although forming is typically associated withinsulators within an MIM structure, semiconductors are generallypreferable in a memory plug because the low resistive state allows afaster access time. However, those skilled in the art will alsoappreciate that very thin insulators can be conductive due to chargetunneling and certain types of insulators, when placed in contact withcertain electrodes, will have an appropriate work function at theelectrode/insulator interface that allows charge injection.

Elemental semiconductors include antimony, arsenic, boron, carbon,germanium, selenium, silicon, sulfur, and tellurium. Semiconductorcompounds include gallium arsenide, indium antimonide, and the oxides ofmost metals. Additionally, some perovskites also exhibit semiconductiveproperties.

Concluding Remarks

Although the invention has been described in its presently contemplatedbest mode, it is clear that it is susceptible to numerous modifications,modes of operation and embodiments, all within the ability and skill ofthose familiar with the art and without exercise of further inventiveactivity. Accordingly, that which is intended to be protected by LettersPatent is set forth in the claims and includes all variations andmodifications that fall within the spirit and scope of the claim.

1. A memory comprising: a substrate including address lines and activecircuitry, the active circuitry including sensing circuitry and addressdecoding circuitry, the address decoding circuitry in communication withthe address lines; addressable two-terminal memory plugs that areactivated by the address decoding circuitry, each two-terminal memoryplug operable to be reversibly written to a first resistive state at afirst write voltage, reversibly written to a second resistive state at asecond write voltage, and have its resistive state determined at a readvoltage; and at least one reference cell that contributes to a referencelevel, wherein the sensing circuitry is operative to produce an outputthat is a function of the resistive state of the activated two-terminalmemory plug and the reference level, wherein the at least one referencecell is similar in structure to the two-terminal memory plugs, whereinthe addressable two-terminal memory plugs and the at least one referencecell are positioned over the substrate.
 2. The memory of claim 1,wherein the reference level has an effective resistance that isintermediate to the first resistive state and the second resistivestate.
 3. A memory comprising: a substrate including address lines andactive circuitry, the active circuitry including sensing circuitry andaddress decoding circuitry, the address decoding circuitry incommunication with the address lines; addressable two-terminal memoryplugs that are activated by the address decoding circuitry, eachtwo-terminal memory plug operable to be reversibly written to a firstresistive state at a first write voltage, reversibly written to a secondresistive state at a second write voltage, and have its resistive statedetermined at a read voltage; and a reference level, wherein the sensingcircuitry is operative to produce an output that is a function of theresistive state of the activated two-terminal memory plug and thereference level, and wherein the addressable two-terminal memory plugsare positioned over the substrate.
 4. A method of programming areference cell, comprising: providing a substrate that includes activecircuitry, a two-terminal memory array positioned over the substrate,and at least one reference cell positioned outside of the two-terminalmemory array and over the substrate; and applying a programming voltageacross a first terminal and a second terminal of the at least onereference cell, the applying operative to change a programmableresistance of the at least one reference cell to a desired value.